The present invention relates to an apparatus for continuously producing a tube having helical grooves in its inner surface and useful chiefly as a refrigerant tube for heat exchangers.
Tubes having helical grooves in the inner surface, when used as refrigerant tubes, assure heat exchange between the refrigerant passing through the tube and the outside air with a higher efficiency than is achieved by usual tubes, so that these grooved tubes are advantageous for producing heat exchangers of reduced size with reduced quantities of materials. An apparatus for producing such a tube internally having helical grooves has already been disclosed in Unexamined Japanese patent Publication No. 167029/73. This conventional apparatus comprises a tube twisting rotary frame, a pay-off drum mounted on the rotary frame for paying off a tube having straight grooves in its inner surface, a pair of catching rollers provided at the front end of the rotary frame for holding the paid-off tube, a pair of rollers arranged in front of the catching rollers for preventing cross sectional deformation of the tube, tubular means disposed in front of the deformation preventing rollers for correcting the cross section of the tube, and a capstan composed of front and rear two winding drums for winding the tube thereon after the tube has passed between the opposed rollers of the pairs and through the tubular means. The straight-grooved tube is twisted between the front winding drum and the catching rollers by the rotation of the rotary frame, whereby the straight grooves are deformed to helical grooves.
The smaller the distance over which the tube is twisted, the better, since if the distance is large and if the tube wall is eccentric, the twisting force will act concentrically on the thin wall portion, possibly flattening the tube of circular cross section. This tendency becomes more pronounced when the twisting angle is greater. With the conventional apparatus, the tube is twisted between the front winding drum and the catching rollers, so that the distance over which the tube is twisted is great. While the front winding drum constitutes the capstan along with the rear winding drum, the capstan need not always comprise two winding drums but may comprise a single winding drum. Accordingly, it may be attempted to reduce the distance by eliminating the front winding drum and also eliminating the deformation preventing rollers in front of the catching rollers and the correcting means in front of the preventing rollers. However, the reduction in the tube twisting distance is limited since the winding drum is diametrically much larger than the catching rollers.
The conventional apparatus has another problem. The catching rollers are intended to prevent the twisting action on the tube from being transmitted toward the pay-off drum. Nevertheless, if the tube is nipped too strongly for this purpose, the tube will not be paid off smoothly, whereas if the tube is nipped too loosely to pay off the tube smoothly, the tube twisting force will be delivered toward the pay-off drum. Thus, it has been impossible for the catching rollers to prevent transmission of the tube twisting action toward the pay-off drum while assuring smooth paying-off of the tube.